The disclosure of Japanese Patent Application No. 2001-266802 filed on Sep. 4, 2001, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a method and an apparatus for controlling a synchronous motor which is suitably used to shift an automatic transmission.
2. Description of the Related Art
For shifting an automatic transmission according to an operation of a shift lever by an operator of an automotive vehicle, for instance, there is known a shifting apparatus which uses as a drive power source a synchronous motor such as a switched reluctance motor or a brushless DC motor.
Unlike a common shifting apparatus operated directly by an operating force applied by the operator to the shift lever to shift the automatic transmission, the shifting apparatus of the kind described above does not require a shifting mechanism to be mechanically connected to the shift lever, and is therefore free of restrictions in the arrangement of its components on the automotive vehicle, leading to a relatively high degree of freedom in its design. The shifting apparatus in question is further advantageous for its relatively easy installation on the automotive vehicle.
In the synchronous motor, the angular position of the rotor is detected by an angular position sensor mounted on the rotor shaft, and stator coils corresponding to the detected angular position of the rotor are energized or excited to rotate the rotor about the axis of the rotor shaft.
Generally, a synchronous motor is controlled by PWM (pulse-width modulation). However, the PWM control of the synchronous motor is difficult unless the resolution of the angular position sensor used to detect the angular position of the rotor is sufficiently high, and accordingly suffers from a problem that the angular position sensor is expensive. The PWM control controls an output voltage by changing a duty ratio of a pulse signal having a predetermined constant period, according to a level of an input signal.
It is also noted that an increase in the switching frequency causes a delay in the moment of initiation of a flow of an electric current through the coils within the synchronous motor, due to an influence of an inductance of the coils. This delay is caused by the characteristic of each coil in preventing a change in the voltage between the opposite ends of the coil. Due to this characteristic of the coil, an increase in the speed of switching the excitation phase of the motor above a certain value, as a result of an increase in the rotating speed of the synchronous motor, will cause a phase difference producing a certain time interval between the moment of initiation of the excitation of the coils and the moment of initiation of a flow of the electric current through the coils. To minimize this phase difference, the synchronous motor is generally controlled by so-called xe2x80x9cvector controlxe2x80x9d which uses a speed sensor for detecting the rotating speed of the synchronous motor, and a current sensor for detecting an amount of delay in the moment of initiation of a flow of the electric current with respect to a switching action, to determine the advance angle of the synchronous motor upon excitation of the coils. In this vector control, the synchronous motor is controlled by controlling the amount of electric current by the PWM control or PAM (pulse-amplitude modulation) control.
However, the conventional vector control requires the control circuit to be provided with the current sensor, and current smoothing condenser and coils, which increases the number of required components, causing problems of increased constructional complexity, size and cost of manufacture of the control circuit. Thus, the known control apparatus for the synchronous motor is not suitably applicable to a system which is required to be inexpensive and small-sized.
The present invention was made to solve the problems described above. It is an object of the prevent invention to provide a method and an apparatus for controlling a synchronous motor, which permits effective reduction in the cost of manufacture and size of the control circuit.
Another object of the present invention is to provide a method and an apparatus for controlling the synchronous motor, which permit reduction of the number of required components and constructional simplification of the control circuit, and ensures increased durability and operating reliability of the control circuit.
A further object of the present invention is to provide a method and an apparatus for controlling the synchronous motor, which permits the synchronous motor to be controlled without the occurrence of an out-of-synchronization state, even in the presence of a variation in the load acting on the synchronous motor.
The objects indicated above may be achieved according to one aspect of this invention, which provides a method of controlling a synchronous motor having a rotor, and a stator which produces a magnetic flux for rotating the rotor about an axis thereof, the method comprising the steps of detecting an angular position and a rotating speed of the rotor; estimating a phase difference between a voltage and an electric current of the synchronous motor, on the basis of the detected rotating speed of the rotor; and adjusting an advance angle of the rotor upon excitation of the synchronous motor as a result of a comparison of the detected rotating speed of the rotor with a target value, to thereby control the rotating speed and direction of the rotor.
The rotating speed of the rotor can be represented by the advance angle, and the rotating speed and the advance angle have a relationship such that the rotating speed increases and decreases with the advance angle. Accordingly, the rotating speed of the rotor can be controlled to a target value, by reducing the advance angle when the actual rotating speed is higher than the target value, and increasing the advance angle when the actual rotating speed is lower than the target value. Therefore, the control circuit need not be provided with a current sensor and a current smoothing condenser and coils, and is therefore available at a lower cost with a reduced number of required components, and is small-sized. Further, the control circuit does not require an angular position sensor to have a high resolution. It is noted that synchronous motors of different types have different relationships between the rotating speed and advance angle of the rotor. To control the specific synchronous motor, therefore, the specific relationship must be obtained by experimentation, for example.
The above objects may also be achieved according to another aspect of this invention, which provides a method of controlling a synchronous motor having a rotor, and a stator which produces a magnetic flux for rotating the rotor about an axis thereof, the method comprising the steps of detecting an angular position and a rotating speed of the rotor; estimating a phase difference between a voltage and an electric current of the synchronous motor, on the basis of the detected rotating speed of the rotor; and adjusting an advance angle of the rotor upon excitation of the synchronous motor, as a result of a comparison of the detected rotating speed of the rotor with an estimated value, whereby the rotating speed and direction of the rotor are controlled.
In the control method for controlling the synchronous motor according to the second aspect of the invention, the angular position and rotating speed of the rotor of the synchronous motor are detected, and the phase difference between the voltage and electric current of the synchronous motor is estimated on the basis of the detected rotating speed of the rotor. Further, the estimated value of the rotating speed of the rotor is obtained according to the relationship between the detected rotating speed and the advance angle of the rotor which corresponds to the estimated phase difference. The detected rotating speed is compared with the thus obtained estimated value, to determine whether the output torque of the synchronous motor is larger or smaller than an optimum value. Thus, the advance angle of the rotor upon excitation of the synchronous motor can be adjusted, so that the synchronous motor can be controlled, without the occurrence of an out-of-synchronization state, even in the presence of a variation in the load acting on the synchronous motor. Therefore, the control circuit is need not be provided with a current sensor and a current smoothing condenser and coils, and is therefore available at a lower cost with a reduced number of required components, and is small-sized. Further, the control circuit does not require an angular position sensor to have a high resolution.
The objects indicated above may also be achieved according to a further aspect of this invention, which provides an apparatus for controlling a synchronous motor having a rotor, and a stator which produces a magnetic flux for rotating the rotor about an axis thereof, comprising a motor controller operable to control the synchronous motor, and an angular position detector operable to detect an angular position and a rotating speed of the rotor, and wherein the motor controller estimates a phase difference between a voltage and an electric current of the synchronous motor, on the basis of the rotating speed of the rotor detected by the angular position detector, and controls the rotating speed and direction of the rotor, by adjusting an advance angle of the rotor upon excitation of the synchronous motor.
In the apparatus according to the above-described aspect of the invention for controlling the synchronous motor, the motor controller estimates a phase difference between the voltage and electric current of the synchronous motor, on the basis of the rotating speed of the rotor detected by the angular position detector, and controls the rotating speed and direction of the rotor, by adjusting an advance angle of the rotor upon excitation of the synchronous motor. The rotating angle of the rotor can be represented by the advance angle of the rotor, and the rotating speed and the advance angle have a relationship that the rotating speed increases and decreases with the advance angle. Accordingly, the advance angle of the rotor upon excitation of the synchronous motor can be adjusted, by controlling the rotating speed of the rotor to a target value or an estimated value, as a result of comparison of the detected actual rotating speed of the rotor with the target value or the estimated value, or by taking into account the output torque of the synchronous motor obtained as a result of comparison of the detected actual rotating speed with the estimated value. Therefore, the control circuit need not be provided with a current sensor and a current smoothing condenser and coils, and is therefore available at a lower cost with a reduced number of required components, and is small-sized. Further, the control circuit does not require an angular position sensor to have a high resolution. It is noted that synchronous motors of different types have different relationships between the rotating speed and advance angle of the rotor. To control the specific synchronous motor, therefore, the specific relationship is required to be obtained by experimentation, for example.